Metal oxides such as lithium metal oxides have found utility in various applications. For example, lithium metal oxides have been used as cathode materials in lithium secondary batteries. Lithium and lithium ion batteries can be used for large power applications such as for electric vehicles. In this specific application, lithium or lithium ion cells are put in series to form a module. In the event that one or more of the cells in the module fails, the rest of the cells become overcharged resulting possibly in explosion of the cells. Therefore, it is important that each cell is individually monitored and protected against overcharging.
The most attractive materials for use as cathode materials for lithium ion secondary batteries have been LiCoO.sub.2, LiNiO.sub.2, and LiMn.sub.2 O.sub.4. However, although these cathode materials are attractive for use in lithium ion secondary batteries, there are definite drawbacks associated with these materials. One of the apparent benefits for using LiNiO.sub.2 and LiCoO.sub.2 as cathode materials is that these lithium metal oxides have a theoretical capacity of 275 mA.multidot.hr/g. Nevertheless, the full capacity of these materials cannot be achieved in practice. In fact, for pure LiNiO.sub.2 and LiCoO.sub.2, only about 140-150 mA.multidot.hr/g can be used. The further removal of lithium by further charging (overcharging) the LiNiO.sub.2 and LiCoO.sub.2 material degrades the cycleability of these materials by moving nickel or cobalt into the lithium layers. Furthermore, the further removal of lithium causes exothermic decomposition of the oxide in contact with the organic electrolyte under heated conditions which poses safety hazards. Therefore, lithium ion cells using LiCoO.sub.2 or LiNiO.sub.2 are typically overcharge protected.
LiCoO.sub.2 and LiNiO.sub.2 have additional disadvantages when used in lithium ion batteries. Specifically, LiNiO.sub.2 raises safety concerns because it has a sharper exothermic reaction at a lower temperature than LiCoO.sub.2. As a result, the charged end product, NiO.sub.2, is unstable and can undergo an exothermic decomposition reaction releasing O.sub.2. See Dahn et al, Solid State Ionics, Vol. 69, 265 (1994). Accordingly, pure LiNiO.sub.2 is generally not selected for use in commercial lithium-ion batteries. Additionally, cobalt is a relatively rare and expensive transition metal, which makes the positive electrode expensive.
Unlike LiCoO.sub.2 and LiNiO.sub.2, LiMn.sub.2 O.sub.4 spinel is believed to be overcharge safe and is a desirable cathode material for that reason. Nevertheless, although cycling over the full capacity range for pure LiMn.sub.2 O.sub.4 can be done safely, the specific capacity of LiMn.sub.2 O.sub.4 is low. Specifically, the theoretical capacity of LiMn.sub.2 O.sub.4 is only 148 mA.multidot.hr/g and typically no more than about 115-120 mA.multidot.hr/g can be obtained with good cycleability. The orthorhombic LiMnO.sub.2 and the tetragonally distorted spinel Li.sub.2 Mn.sub.2 O.sub.4 have the potential for larger capacities than is obtained with the LiMn.sub.2 O.sub.4 spinel. However, cycling over the full capacity range for LiMnO.sub.2 and Li.sub.2 Mn.sub.2 O.sub.4 results in a rapid capacity fade.
Various attempts have been made to either improve the specific capacity or safety of the lithium metal oxides used in secondary lithium batteries by doping these lithium metal oxides with other cations. For example, cobalt cations have been used to dope LiNiO.sub.2. Nevertheless, although the resulting solid solution LiNi.sub.1-x Co.sub.x O.sub.2 (0.ltoreq.X.ltoreq.1) may have somewhat improved safety characteristics over LiNiO.sub.2 and larger useful capacity below 4.3 V versus Li than LiCoO.sub.2, this solid solution still has to be overcharge protected just as LiCoO.sub.2 and LiNiO.sub.2.
One alternative has been to dope LiNiO.sub.2 with ions that have no remaining valence electrons thereby forcing the material into an insulator state at a certain point of charge and protecting the material from overcharge. For example, Ohzuku et al. (Journal of Electrochemical Soc., Vol. 142, 4033 (1995)) describe that the use of Al.sup.3+ as a dopant for lithium nickelates to produce LiNi.sub.0.75 Al.sub.0.25 O.sub.4 can result in improved overcharge protection and thermal stability in the fully charged state as compared to LiNiO.sub.2. However, the cycle life performance of this material is unknown. Alternatively, U.S. Pat. No. 5,595,842 to Nakare et al. demonstrates the use of Ga.sup.3+ instead of Al.sup.3+. In another example, U.S. Pat. No. 5,370,949 to Davidson et al. demonstrates that introducing chromium cations into LiMnO.sub.2 can produce a tetragonally distorted spinel type of structure which is air stable and has good reversibility on cycling in lithium cells.
Although doping lithium metal oxides with single dopants has been successful in improving these materials, the choice of single dopants which can be used to replace the metal in the lithium metal oxide is limited by many factors. For example, the dopant ion has to have the right electron configuration in addition to having the right valency. For example, Co.sup.3+, Al.sup.3+, and Ga.sup.3+ all have the same valency but Co.sup.3+ can be oxidized to Co.sup.4+ while Al.sup.3+, and Ga.sup.3+ cannot. Therefore doping LiNiO.sub.2 with Al or Ga can produce overcharge protection while doping with cobalt does not have the same effect. The dopant ions also have to reside at the correct sites in the structure. Rossen et al (Solid State Ionics Vol. 57, 311 (1992)) shows that introducing Mn into LiNiO.sub.2 promotes cation mixing and therefore has a detrimental effect on performance. Furthermore, one has to consider the ease at which the doping reaction can be carried out, the cost of the dopants, and the toxicity of the dopants. All of these factors further limit the choice of single dopants.
In addition to these factors, it is also desirable that the doped lithium metal oxide has a high usable reversible capacity and good cycleability to maintain this reversible capacity during cycling. As mentioned above, LiNiO.sub.2 and LiCoO.sub.2 have usable reversible capacities in the range of 140-150 mA.multidot.hr/g because of their low thermal stability. Moreover, LiMn.sub.2 O.sub.4 can generally only be operated at 115-120 mA.multidot.hr/g with good cycleability. Therefore, there is a need in the art to produce a doped lithium metal oxide that exhibits an improved reversible capacity and good cycleability while maintaining thermal stability.